Suppliers of photolithographic equipment for the semiconductor industry are actively performing research and development programs directed toward the use of Q-switched (QS) or Q-switched cavity dumped (QSCD) CO2 laser oscillator/amplifier systems for use as the optical pump source in the generation of extreme ultra-violet (EUV) radiation via plasma generation. This course of action has been selected because these lasers have a high peak power output, such as equal to or greater than 1 KW for Q-switched (QS) lasers and about 10 KW for Q-Switched Cavity Dump (QSCD) lasers. These lasers have short pulse durations, such as 0.1 to 2 microseconds for QS lasers and 10 to 30 ns for QSCD lasers. These lasers also have reasonable oscillator output average powers, such as on the order of about 20 to 50 W, in single mode operation, and have high pulse repetition rates, such as ≧100 KHz. These lasers can run in single mode operation, and can have a relatively lower cost than solid state laser sources. Output powers exceeding 35 KW have been obtained with gas flowing CO2 lasers which are suitable for high power amplifier applications.
Another application for QS and QSCD CO2 lasers is in remote sensing systems using coherent laser radar (LADAR) technology, where range and Doppler information are of interest.
Present CO2 QS and QSCD lasers have variations in pulse peak power with time, and considerable turn-on time jitter from pulse to pulse. The pulse amplitude and turn-on time jitter variations occur primarily due to statistics associated with the laser pulses building up from noise, from the drifting of the axial modes of the resonator across the oscillating CO2 gain curve with temperature, as well as from variations in the discharge. These problems are common to solid state lasers as well. Switching the laser wavelength from pulse to pulse to the next highest gain line amongst the numerous rotational lines of the CO2 molecule during the Q-switching process is also common in QS or QSCD CO2 lasers. This line switching is primarily believed to be caused by the drifting of the axial modes of the laser resonator, with temperature and discharge variations, across the vibrational/rotational lines of the CO2 molecules. During laser operation, these lines have varying amounts of inverted population (i.e. gain). This frequency switching from pulse to pulse also contributes to the peak pulse power fluctuations, pulse to pulse time jitter, and changes in pulse width of present QS and QSCD CO2 lasers. These pulse to pulse variations present in existing systems can result in system performance variations in the amount of deep ultra-violet generation for photolithography applications, the maximum detectable range of laser radars systems, and the accuracy of the velocity of a target that can be measured with heterodyned CO2 laser radar systems. Such variations in system performance are not acceptable.